Method for performing handover procedure and creating data

ABSTRACT

A method is described of performing handover by a mobile terminal from a source base station to a target base station, the mobile terminal having a Radio Resource Control (RRC) layer and a Radio Link Control (RLC) layer. The RRC layer receives a handover command. The RLC layer receives an indication associated with the handover command. Upon receiving the indication, at least one RLC service data unit (SDU) from at least one protocol data unit (PDU) having a sequence number that is less than a variable is reassembled. The variable indicates a sequence number following a highest sequence number among at least one PDU received out of sequence by the RLC layer. The reassembled at least one RLC SDU is delivered to an upper layer of the mobile terminal, and at least one remaining PDU that could not be reassembled into at least one RLC SDU is discarded.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 12/323,902 filed on Nov. 26, 2008, which claims priority toKorean Patent Application No. 10-2008-0008165, filed on Jan. 25, 2008.The entire contents of all of the above applications are herebyincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless communications and, moreparticularly, to a method for performing a handover procedure andcreating data.

2. Discussion of the Related Art

A cellular scheme is a concept proposed to overcome the limitation ofservice area, and frequency and subscriber accommodation capacity. Itprovides a speech area (call area, or a communication range) by changinga single base station of a high output into a plurality of base stationsof a low output. Namely, a mobile communication service area is dividedinto a plurality of small cells, and different frequencies are allocatedto neighbor cells while the same frequency band is allocated to cellswhich are so away from each other that no interference is generatedtherebetween to thus spatially re-use frequencies.

Handover (or handoff) refers to a function according to which when auser equipment (UE) moves and is released from a current communicationservice base station (referred to as ‘serving cell’, hereinafter) andenters a neighbor cell of an adjacent communication service area, the UEis automatically tuned with a new traffic channel of the adjacentcommunication service area and maintains its call state. That is, whenthe UE is communicating with a particular base station and if a signalstrength of the particular base station (referred to as ‘source basestation, hereinafter) weakens, the UE is linked to a neighbor basestation (referred to as ‘target base station, hereinafter). Whenhandover is supported, a call interruption, that may be otherwisegenerated when the UE moves to a neighbor cell, can be resolved.

Handover may be defined as three separated steps, a handoverpreparation, a handover execution, and a handover completion.

The UE measures a signal transmitted from the serving cell, and if achannel quality is drops to below a certain threshold value, the UEtransmits corresponding information to the source base station through ameasurement report. Then, the source base station determines handoveraccording to the measurement report. Upon determining handover, thesource base station a handover request to the target base station andthen receives a handover request ACK from the target base station. Thesource base station informs the UE about a start of handover by sendinga handover command. The processes up till now belong to the handoverpreparation.

After the handover preparation, the UE moves to the target base stationthrough the handover execution and the handover completion processes. Atthis time, the source base station stops downlink data transmission of aPDCP (Packet Data Convergence Protocol) layer and forwards downlink datato the target base station.

In this respect, however, an RLC layer and a PDCP layer of the UE in thedownlink transmission and an RLC layer and a PDCP layer of the basestation in the uplink transmission do not know the handover executionand completion processes. Accordingly, the RLC layers cannot help buttransfer SDUs (Service Data Units) in sequence regardless of handover.This is the same even when the RLC layers cannot transfer the SDUs tothe PDCP layers due to a blank of PDUs corresponding to some sequencenumbers. The PDCP layers also cannot forward nor reorder downlink oruplink data.

Due to such a restriction, if handover is completed with a blank of aPDU, other PDUs following the blank PDU are all removed. The downlinkdata removed at the RLC layer of the UE can be restored as the PDCPlayer of the UE requests re-transmission of the downlink data from thePDCP layer of the target base station after handover is completed. Andthe data removed at the RLC layer of the base station can be restored asthe PDCP layer of the target base station requests re-transmission ofthe data after the handover is completed.

However, because such restoration requests restoration between the PDCPlayer of the UE and that of the target base station, namely, requeststhe re-transmission in the wireless communication, radio resources arewasted, and because the restoration time is delayed, the transmissionefficiency is degraded.

Thus, in order to solve the problem, a handover executing method anddata creating method allowing data transmission/reception between theRLC layer and the PDCP layer while the handover is being performed isrequested.

SUMMARY OF THE INVENTION

The present invention provides a method for performing handoverprocedure and creating data. According to an embodiment of theinvention, a method and device for performing handover by a UE from asource base station to a target base station is provided. The methodincludes receiving at least one data block from the source base station,receiving a handover command from the source base station, assemblingthe at least one data block out of sequence regardless of the receptionorder to create a reassembled data block, and transmitting a handoverconfirmation to the target base station.

According to another embodiment of the invention, a method and devicefor performing handover by a source base station to a target basestation over a UE is provided. The method includes transmitting ahandover command to the UE, assembling at least one data blocktransmitted from the UE out of sequence regardless of the receptionorder to create a reassembled data block, and forwarding the reassembleddata block to the target base station.

According to still another embodiment of the invention, a method anddevice for creating a data block by an RLC layer is provided. The methodincludes receiving at least one PDU, receiving a message indicatingreassembling the at least one PDU out of sequence regardless of thereception order, and reassembling the at least one PDU out of sequenceto create an SDU.

In the present invention, a lower layer is informed about a handovertime point, so that the lower layer can generate data blocks out ofsequence and transmits them to an upper layer, to thereby guarantee atransfer of data without a loss although handover is performed. Becausedata is transmitted without a loss, overhead due to re-transmission dataand overhead due to an additional control signal can be reduced, and aprocessing gain can be obtained.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram showing a conventional wirelesscommunication system.

FIG. 2 is a schematic block diagram showing a conventional functionalsplit between an E-UTRAN and an EPC.

FIG. 3 is a schematic block diagram showing elements of a conventionalUE.

FIG. 4 is a schematic block diagram showing a user plane of aconventional radio interface protocol.

FIG. 5 is a schematic block diagram showing a control plane of theconventional radio interface protocol.

FIGS. 6 and 7 are flow charts illustrating a point of time for reportinga start of handover execution to an RLC layer according to an embodimentof the present invention.

FIGS. 8 and 9 are flow charts illustrating the process of a method forreporting handover to the RLC layer and a PDCP layer according to anembodiment of the present invention.

FIG. 10 is a flow chart illustrating the process of a method forreporting handover completion according to an embodiment of the presentinvention.

FIG. 11 is a flow chart illustrating the process of a method forre-using the RLC layer when handover fails according to an embodiment ofthe present invention.

FIG. 12 is a flow chart illustrating the process of a method foroperating the RLC layer which has been reported on handover executionaccording to an embodiment of the present invention.

FIG. 13 is a view showing a method for creating and delivering SDUs outof sequence according to one embodiment of the present invention.

FIG. 14 is a flow chart illustrating a method for creating anddelivering SDUs out of sequence according to another embodiment of thepresent invention.

FIG. 15 is a view showing a method for creating and delivering SDUs outof sequence in FIG. 14 according to one embodiment of the presentinvention.

FIG. 16 is a view showing a method for creating and delivering SDUs outof sequence in FIG. 14 according to another embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic block diagram showing a wireless communicationsystem. It may be a network structure of an E-UMTS (Evolved-UniversalMobile Telecommunications System). The E-UMTS system may be an LTE (LongTerm Evolution) system. The wireless communication system can be widelydeployed to provide a variety of communication services, such as voices,packet data, etc.

Referring to FIG. 1, an E-UTRAN (Evolved-UMTS Terrestrial Radio AccessNetwork) includes a base station (BS) 20. user equipment (UE) 10 may befixed or have mobility, and may be referred to as another terminology,such as a mobile station (MS), a user terminal (UT), a subscriberstation (SS), a wireless device, etc. The BS 20 generally refers to afixed station that communicates with the UE 10 and may be called anotherterminology, such as an NB (Node-B), an eNB (evolved-Node B), a BTS(Base Transceiver System), an access point, etc. There are one or morecells within the coverage of the BS 20. An interface may be used foruser traffic or control traffic between BSs 20. Hereinbelow, “downlink”refers to communication from the BS 20 to the UE 10, and “uplink” refersto communication from the UE 10 to the BS 20.

The BS 20 provides end points of a user plane and a control plane to theUE 10. The BSs 20 may be connected via an X2 interface, and a meshednetwork structure in which the X2 interface always exists may beprovided between neighbor BSs 20.

FIG. 2 is a schematic block diagram showing a functional split betweenan E-UTRAN and an EPC.

With reference to FIG. 2, shaded blocks (with oblique lines) representradio protocol layers, and empty blocks represent functional entities ofthe control plane.

The BS performs the following functions: (1) radio resource management(RRM) function such as radio bearer (RB) control, radio admissioncontrol, connection mobility control, dynamic resource allocation to theUE; (2) IP (Internet Protocol) header compression and decryption of userdata stream; (3) routing of user plane data to a serving gateway (S-GW);(4) scheduling and transmission of a paging message; (5) scheduling andtransmission of broadcast information; and (6) measurement for mobilityand scheduling and establishing a measurement report.

An MME performs the following functions: (1) distribution of pagingmessages to BSs; (2) security control; (3) idle state mobility control;(4) S bearer control; (5) ciphering and integrity protection of NAS(Non-Access Stratum) signaling.

The S-GW performs the following functions: (1) termination of a userplane packet with respect to paging; and (2) user plane switching tosupport UE mobility.

Meanwhile, layers of a radio interface protocol between the UE and thebase station include a physical layer, a MAC (Medium Access Control)layer, an RLC (Radio Link Control) layer, a PDCP (Packet DataConvergence Protocol) layer, an RRC (Radio Resource Control) layer.

The layers of the radio interface protocol may be divided into a firstlayer L1, a second layer L2, and a third layer L3 based on the threelower layers of an open system interconnection (OSI) standard modelwidely known in communication systems. Compared with the OSI model, thephysical layer corresponds to the first layer L1, the upper MAC layerand the RLC layer correspond to the second layer L2, and the RRC layercorresponds to the third layer L3. The physical layer belonging to thefirst layer provides an information transfer service using a physicalchannel, and the radio RRC layer positioned at the third layer L3 servesto control radio resources between the UE and the network.

The layer structure of the radio interface protocol may be applied inthe same manner to the UE and the E-UTRAN. In the UE, all the protocolsmay belong to a single entity, while in the E-UTRAN, the protocols maybe distributed by respective network configuration elements.

Data transmitted by such entire protocol structure may be divided intotwo regions of a user plane and a control plane according to a type ofthe data. The user plane is a region where traffic information of a usersuch as voice or IP packets are transmitted, and the control plane is aregion where control information such as a network interface,maintaining or management of a call, or the like. Data transferred bythe RRC layer is included in the control plane. The RLC layer may belongto the user plane or the control plane according to a type of aconnected upper layer. Namely, if the RLC layer is connected with theRRC layer, it may belong to the control plane, and in other cases, theRLC layer may belong to the user plane.

FIG. 3 is a schematic block diagram showing elements of a UE. The UE 50includes a processor 51, a memory 52, an RF unit 53, a display unit 54,and a user interface unit 55. The processor 51 includes the layers ofthe radio interface protocol and provides the control plane and the userplane. Functions of the layers may be implemented via the processor 51.The memory 52 is connected with the processor 51 and stores a UE drivingsystem, an application and a general file. The display unit 54 displaysvarious information of the UE and may be formed by using the well knowelements such as an LCD (Liquid Crystal Display), an OLED (Organic LightEmitting Diode), or the like. The user interface unit 55 may beconfigured by combining well known user interfaces such as a keypad or atouch screen. The RF unit 53 is connected with the processor andtransmits and/or receives a radio signal.

FIG. 4 is a schematic block diagram showing the user plane of the radiointerface protocol. FIG. 5 is a schematic block, diagram showing thecontrol plane of the radio interface protocol. It shows the structure ofthe radio interface protocol between the UE and the E-UTRAN. The userplane is a protocol stack for transmitting user data, and the controlplane is a protocol stack for transmitting a control signal.

With reference to FIGS. 4 and 5, the physical layer, as a first layer,provides an information transfer service to an upper layer by using aphysical channel. The physical layer is connected to an upper layercalled a medium access control (MAC) layer via a transport channel, anddata is transferred between the MAC layer and the physical layer via thetransport channel. Meanwhile, between different physical layers, namely,between a physical layer of a transmitting side and that of a receivingside, data is transferred via the physical channel.

Techniques such as data multiplexing, channel coding, spreading,modulation, or the like, are applied for the physical layer. In thewireless environment, radio signals change frequently according to themovement of the UE or a surrounding environment, so various methods forcorrecting them are required.

A radio data link layer corresponding to the second layer includes a MAClayer, an RLC layer, and a PDCP layer. The MAC layer of the secondlayer, which handles mapping between a logical channel and a transportchannel, selects a proper transport channel to transmit data receivedfrom the RLC layer, and adds required control information to a header ofa MAC PDU (Protocol Data Unit). A mapping relationship between thelogical channel and the transparent channel will be described hereafter.

The RLC layer of the second layer is positioned at an upper position ofthe MAC layer and supports reliable data transmissions. The RLC layersegments and concatenates RLC SDUs (Service Data Units) delivered froman upper layer in order to configure data with a size suitable for aradio interface. The RLC layer of a receiver supports a reassemblingfunction of data to restore the original RLC SDUs from received RLCPDUs.

Each RLC entity may operate in a transparent mode (TM), anunacknowledged mode (UM), and an acknowledged mode (AM) according to aprocessing and transmission method of the RLC SDUs. CRC error detectionis performed at the physical layer in all the RLC modes. The results ofthe CRC checking are delivered together with actual data to the RLClayer.

In the transparent mode (TM), the RLC layer delivers an RLC PDU, withoutadding a protocol header thereto, to the MAC layer via the transportchannel. If an RLC PDU has a transmission error, it may be removed orhave an indication that it has an error. The TM may be used when upperlayer data is a streaming type data. In this case, the upper layer datais not segmented, and in a particular case, a segmentation/reassemblingfunction is used limitedly.

In the unacknowledged mode (UM), a re-transmission protocol is not used,so data transfer is not guaranteed. A transmitter deletes data based ona timer without definite signaling, so RLC PDUs, which have not beentransmitted within a particular time, are removed from a transmissionbuffer. The PDUs include sequence numbers (SN), so integrity of the PDUsof an upper layer can be observed. The UM RLC entity is defineduni-directionally because relationship between uplink and downlink isnot required. For example, a user service to which the UM RLC entity canbe applicable includes a cell broadcast service and a VoIP (Voice overInternet Protocol).

The acknowledged mode (AM) uses an ARQ process to correct an error. Ifthe RLC PDU is not properly transferred (e.g., as it exceeds the maximumnumber of re-transmissions or as a transmission time lapses), the RLClayer reports the upper layer accordingly and removes the RLC PDU fromthe buffer. An AM RLC entity, having a re-transmission function,provides a bi-directional service.

For the re-transmission function of the AM, various parameters such as atransmission window (Tx window), a reception window (Rx window), atimer, a counter, a status PDU (or a status report), a polling bit, orthe like, are used. The transmission window is the number of RLC PDUsthat can be transmitted with the maximum number, in a state that astatus PDU is not received from the receiver.

The PDCP layer of the second layer is used only at a packet switchingregion, and may compress a header of an IP packet and transmit the samein order to enhance a transmission efficiency of packet data in a radiochannel.

The RRC layer of the third layer is defined only at the control plane.The RRC layer serves to control a lower layer and exchanges radioresource control information between the UE and the network. Various RRCstates are defined according to a communication state of the UE, and atransition between RRC states is possible as necessary. In the RRClayer, various procedures related to radio resource management such as asystem information broadcast, an RRC connection management procedure, aradio resource control procedure, a security procedure, a measurementprocedure, a mobility management procedure (handover), or the like, aredefined.

A point of time at which handover is reported to a lower layer and itsmethod will now be described. Here, the lower layer refers to a layerwhich is lower to the RRC layer, and the lower layer may refer to theRLC layer or the PDCP layer.

FIGS. 6 and 7 are flow charts illustrating a point of time for reportinga start of handover execution to the RLC layer according to anembodiment of the present invention. In FIGS. 6 and 7, the points ‘a’ to‘e’ are connected, respectively.

With reference to FIGS. 6 and 7, a source base station (source eNB)transmits a measurement control to the UE (S10). The UE transmits ameasurement report to the source base station (S11). The source eNBdetermines whether to perform handover (S12). When the source eNBdetermines handover, it transmits a handover request to a target basestation (target eNB) (S13). The target eNB performs an admission controlwith respect to the handover request (S14).

When the target eNB admits the handover request, it transmits a handoverrequest ACK to the source eNB (S15). The source eNB transmits a handovercommand to the UE (S16). The processes up till now belong to handoverpreparation.

At the point of time of handover execution, the following step, isreported to the RLC layer of the UE and to the RLC layer of the sourceeNB (S17). When the handover command is transmitted from the source eNBto the UE, the RRC layer of the source eNB reports the handover to thelower RLC layer and PDCP layer. Also, the RRC layer of the UE reportsthe handover to the lower RLC layer and PDCP layer. Upon receiving thereport on the handover, the source eNB and the UE starts out-of-sequenceSDU delivery from the RLC layer to the PDCP layer in the respectivedevices (S18). Here, the out-of-sequence SDUs refer to at least one datablock included in a reception window (or OSD_Window, to be described)among data blocks stored in a reception buffer. The out-of-sequence SDUsare generated by being reassembled regardless of the order in which theyare stored in the reception buffer. In this sense, the out-of-sequenceSDUs are discriminated from in-sequence SDUs, and a data loss can bereduced during the handover through the out-of-sequence SDU delivery.

Also at the point of time of handover execution, the UE is detached froman old cell of the source eNB and attempts synchronization with a newcell of the target eNB, and the source eNB transfers data packets storedin the buffer to the target eNB (S18).

Next, the UE matches synchronization to the target eNB (S19). The targeteNB allocates uplink resources to the UE and performs time alignment(TA) (S20). The UE transmits a handover confirmation to the target eNB(S21). As the handover configuration is transmitted, the handoverexecution is terminated, and the handover completion follows.

Upon receiving the handover confirmation, the target eNB transmits apath switch to an MME. The MME transmits a U-plane update request to aserving GW (S23). The serving GW switches a downlink path (S24) andtransmits a U-plane update response to the MME (S25).

The MME transmits a path switch ACK to the target eNB (S26). The targeteNB transmits a resource release to the source eNB (S27). Upon receivingthe resource release, the RRC layer of the source eNB reports thehandover completion to the lower RLC layer and PDCP layer (S28). Then,the RLC layer of the source eNB flushes a downlink data buffer anddelivers in-transit DL data packets to the target eNB. The source eNBreleases resources (S29), and at this time, the handover procedure isterminated.

In the conventional art, the RLC layer may deliver SDUs in sequence toan upper layer. However, if the in-sequence SDU delivery is performed atthe point of time when handover execution starts in accordance with theconventional art, a data loss may occur. Thus, in the present invention,the RRC layer informs the RLC layer and the PDCP layer about the pointof time of the handover, so that the RLC layer can perform theout-of-sequence SDU delivery during handover in a manner to therebyminimize a data loss. Thus, during the handover, PDCP layer may buffernon-transmitted data or reorder received data.

FIGS. 8 and 9 are flow charts illustrating the process of a method forreporting handover to the RLC layer and the PDCP layer according to anembodiment of the present invention. In FIGS. 8 and 9, the points ‘f’ to‘n’ are connected, respectively.

With reference to FIGS. 8 and 9, the method for reporting handover tothe RLC layer and the PDCP layer may be divided into a method fortransmitting a handover command from the source eNB to the UE, a methodfor informing the second layer L2 of the source eNB about the handoverexecution, and a method for informing the second layer L2 of the UEabout the handover execution. Hereinbelow, the respective processes willbe discriminately described.

The following paragraphs describe the method for transmitting thehandover command. The handover command is made through RRC signalingfrom the RRC layer of the source eNB to that of the UE, so the RRC layerof the source eNB transmits the handover command by delivering aprimitive PDCP AM data request, an L2 message, to the PDCP layer (S101).The PDCP layer of the source eNB delivers a primitive RLC AM datarequest to the RLC layer (S102).

The RLC layer of the source eNB performs a segmentation/concatenationprocess to create RLC AMD data PDUs and transmits them to the RLC layerof the UE (S103). The RLC layer of the UE delivers a primitive RLC AMdata indicator (RLC AM data IND) to the PDCP layer of the UE (S104), andthe PDCP layer of the UE delivers a PDCP AM data indicator (PDCP AM dataIND) to the RRC layer of the UE (S105). In this manner, the handovercommand is transmitted to the RRC layer of the UE.

The RLC layer of the source eNB receives an ACK as an RLC AMD controlPDU from the RLC layer of the UE (S106). The RLC layer of the source eNBcannot know whether content of data transmitted to the UE is thehandover command. Thus, the RLC layer of the source eNB delivers aprimitive RLC AM data CNF to the PDCP layer by using a message unit ID(MUI) between the PDCP layer and the RLC layer and informs about thetransmission confirmation for the MUI (S107).

The PDCP layer of the source eNB delivers L2 ACK, a receptionconfirmation response of the L2 message, to the RRC layer of the sourceeNB (S108). The RRC layer of the source eNB can recognize that the UEreceived the handover command and will enter the handover executionprocedure. The RRC layer of the source eNB may inform the L2 layer aboutthe start of the handover execution by receiving L2 ACK.

The following paragraphs describe the method for informing the secondlayer of the source eNB about the handover execution.

The RRC layer of the source eNB delivers a primitive PDCP handover startindicator (IND) to the PDCP layer of the source eNB (S109). And the RRClayer of the source eNB transmits a primitive CRLC handover startindicator (Primitive CRLC_HO_Start_IND) to every RLC UM entity and RLCAM entity of the UE (S110). Upon receiving the primitive PDCP handoverstart indicator, the PDCP layer of the source eNB stops transmission ofdownlink data with respect to every resource block (RB) of the UE andbuffers the downlink data until when the handover is completed (S111).

Meanwhile, upon receiving the primitive CRLC handover start indicator,the RLC layer of the UE performs out-of-sequence SDU delivery for uplinkdata (S112). Separately, the PDCP layer of the source eNB transmits thebuffered downlink data to the PDCP layer of the target eNB (S113). Thetarget eNB starts buffering the received downlink data (S114).

The RLC layer of the source eNB stops receiving of the data, reassemblesPDUs stored in the reception buffer (Rx buffer) into SDUs, and deliversthe SDUs, to the PDCP layer of the source eNB via a primitiveRLC_OSD_DATA_IND (S115). SDU segments, which have failed to bereassembled into the SDUs, are deleted from the reception buffer, andthe timer, a state variable, or the like, are reset (S116). In thismanner, when the handover fails, the RLC layer can transmit/receivedata.

The PDCP layer of the source eNB transfers the uplink data blocksreceived in sequence to the S-GW (S117), and buffers the uplink datablocks received out of sequence (S118). The PDCP layer of the source eNBforwards the buffered out-of-sequence uplink data blocks to the PDCPlayer of the target eNB (S119). The PDCP layer of the target eNBreorders the in-sequence uplink data blocks and the out-of-sequenceuplink data blocks (S120).

The following paragraphs describe the method within the UE for informingthe second layer of the UE about the handover execution.

When the RRC layer of the UE receives a handover command, it delivers aprimitive PDCP_HO_START_IND to the PDCP layer of the UE (S121). The RRClayer of the UE transmits a primitive CRLC_HO_START_IND to every RLC UMentity and RLC AM entity (S122). The PDCP layer of the UE stopstransmission of uplink data with respect to every resource block andbuffers the uplink data blocks until when the handover is completed(S123). Meanwhile, the RLC layer of the UE performs out-of-sequence SDUdelivery on downlink data (S124).

Upon receiving the primitive CRLC_HO_START_IND from the PDCP layer ofthe UE, the RLC layer of the UE stops receiving downlink data andreassembles at least one PDU, included in a reception window (orOSD_Window) among PDUs stored in the reception buffer, into SDUs. Andthe RLC layer of the UE transfers the reassembled SDUs to the PDCP layerof the UE via a primitive RLC_OSD_DATA_IND (S125). In this case, SDUsegments, which have failed to be reassembled into the SDUs, are deletedfrom the reception buffer, and the timer, a state variable, or the like,are reset (S126). Through resetting, only data is deleted from thereception buffer while the RLC entity remains, and meanwhile, the RLCentity can be deleted through releasing.

The PDCP layer of the UE reorders the out-of-sequence downlink datauntil when the handover is completed (S127), and delivers thein-sequence downlink data to an upper layer (S128).

Thus, because the time point of the handover execution is informed tothe RLC layer of the UE and to the RLC layer of the source eNB, the RLClayers of the UE and the source eNB can deliver their respective uplinkand downlink SDUs, which are assembled out of sequence, to the PDCPlayers of the UE and the source eNB, and thus, a data loss duringhandover can be reduced. In addition, the source eNB can not only informthe target eNB about the time and method of downlink data forwarding butalso reorder and transfer the previously received uplink data to theS-GW to be forwarded to the target eNB.

As mentioned above, the handover procedure is divided into a handoverpreparation phase, a handover execution phase, and a handover completionphase, of which the handover preparation and the handover execution havebeen explained. The following paragraphs describe the method forreporting the handover completion to the RLC layer and the PDCP layer.

FIG. 10 is a flow chart illustrating the process of the method forreporting the handover completion according to an embodiment of thepresent invention. The handover completion is reported to the secondlayer L2 of the source eNB.

With reference to FIG. 10, the RRC layer of the target eNB transmitsresource release to the RRC layer of the source eNB (S201). The RRClayer of the source eNB can recognize that the handover has beensuccessfully performed through the handover completion. As the handoveris successfully completed, the second layer of the source eNB is notnecessary any longer, so it is released. In this regard, if the RLClayer is released when the handover starts, and in this state, if thehandover fails, a new configuration process of the RLC layer would berequired, so the releasing of the RLC layer at the time of startinghandover is not desirous. Consequently, the RRC layer of the target eNBtransmits the resource release so that the second layer L2 of the sourceeNB can be released when the handover is completed.

Upon receiving the resource release, the RRC layer of the source eNBdelivers a primitive PDCP release request to the PDCP layer of thesource eNB to release every PDCP layer with respect to the UE (S202).Also, the RRC layer of the source eNB delivers a primitiveCRLC_Release_Request to the RLC layer of the source eNB to release everyRLC entity of the UE (S203). The RLC layer of the UE releases every RLCentity (S204).

FIG. 11 is a flow chart illustrating the process of a method forre-using the RLC layer when handover fails according to an embodiment ofthe present invention.

With reference to FIG. 11, if the handover fails, the source eNB and theUE should transmit/receive data again. With the handover failing, theRRC layer of the source eNB reports re-start of datatransmission/reception to every radio bearer with respect to the UEthrough a primitive PDCP_HO_Failure_IND (S301). Meanwhile, the RLClayers have been reset, not released, and synchronization is matched atthe both ends, so the RLC layers can immediately perform downlink datatransmission/reception by re-using existing RLC entities without aprimitive (S302, S303).

The following paragraphs describe the method for operating the RLC layerwhich is reported on the handover execution.

FIG. 12 is a flow chart illustrating the process of the method foroperating the RLC layer which is reported on the handover executionaccording to an embodiment of the present invention. Specifically, FIG.12 shows an out-of-sequence SDU delivery operation of the RLC layer of areception end, among operations of the RLC layer. Here, RLC layer of thereception end refers to an RLC layer of a UE or an eNB that receivedata. Namely, RLC layer of the reception end may be the RLC layer of thesource eNB in the uplink data transmission or the RLC layer of the UE inthe downlink data transmission.

With reference to FIG. 12, the process begins when the RLC layer isready to transmit/receive data (DATA_TRANSFER_READY) (5401). The RLClayer receives a primitive CRLC_HO_START_IND from the RRC layer (S402).At this time, the RLC layer stops receiving of data (S403). The RLClayer decodes segmentation header information such as sequenceinformation SI, sequence order SO, length information LI, or the like,to check whether PDUs, at least one data block, can be assembledregardless of the reception order to create reassembled data blocks,namely SDUs (S404).

If the PDUs can be reassembled into SDUs, RLC layer reassembles the PDUsinto SDUs (S405) and delivers the reassembled at least one SDU to thePDCP layer (S406). At this time, the RLC layer is reset by initializingstate variables and the timer except configurable parameters (S407).

If the PDUs are determined not to be reassembled into SDUs, the RLClayer removes the PDUs that cannot be reassembled (S408) and is re-set(S407). Thereafter, in case of handover failure, the RLC layer is readyfor data transmission/reception (DATA_TRANSFER_READY) totransmit/receive data again. Here, in the resetting, a reset PDU is nottransmitted to a counterpart RLC layer, it is not necessary totransition to a reset pending state. That is, because the RLC layers ofthe UE and the source eNB are all reset according to the handovercommand, there is no need to match synchronization between the RLClayers of the both ends. In resetting, the RLC layer of the receptionend as well as that of the transmission end is reset.

The following paragraphs describe the method for performingout-of-sequence SDU delivery by the RLC upon receiving a report on thehandover execution.

FIG. 13 describes the method for performing out-of-sequence SDUsdelivery according to one embodiment of the present invention.Specifically, FIG. 13 shows the case where re-transmission of datawithin the reception buffer of the RLC layer and reassembling of datablocks are processed by using the reception window (Rx_Window).

With reference to FIG. 13, the PDUs, data blocks, corresponding tosequential numbers 0, 1, 3, 4, and 5 are stored, and the size of thereception window (Rx_Window) is 10. The reception buffer corresponds toa reception buffer of the RLC layer after the primitiveCRLC_HO_START_IND is received from the RRC layer. The reception buffermay be that of the RLC layer of the UE or that of the RLC layer of theeNB. VR(R) and VR(H) are state variables, and the reception window is aconfigurable parameter. In the reception buffer, the RLC entity mayreorder the PDUs, data blocks, by the size of the reception window andreassemble them into at least one SDU, a combination of data blocks.

The VR(R) indicates a sequence number (SN) which the RLC entity expectsto receive in sequence, and the VR(H) indicates an SN which is 1 largerthan the largest one of SNs. The PDUs corresponding to SNs 0 and 1 forman SDU, one reassembled data block, and the PDUs corresponding to SNs 4and 5 form an SDU, another reassembled data block. The two reassembleddata blocks, SDUs, are delivered from the RLC layer to the PDCP layer.Hereinafter, the reassembled data blocks created by assembling one ormore data blocks within the reception window regardless of the order ofthe SNs of the data blocks are called reassembled SDUs and the datablocks are called PDUs.

After receiving the CRLC_HO_START_IND, if the PDUs of the SNs 0, 1, 3,4, and 5 have been stored in the reception buffer, the RLC layer stopsreceiving of data, decodes segmentation headers to reassemble all thePDUs that can be reassembled into SDUs, and delivers the reassembledSDUs to the PDCP layer.

The RLC layer may transmit the two reassembled SDUs to the PDCP layer,separately, or may transmit all the SDUs to the PDCP layer collectivelyat a time. When the SDUs are transmitted separately, information aboutthe last SDU that is finally transmitted among the SDUs, should beprovided to the PDCP layer. When the SDUs are transmitted collectively,there is no need to inform the PDCP layer about the last SDU, so thetransmission efficiency can be improved.

FIG. 14 is a flow chart illustrating a method for creating anddelivering SDUs out of sequence according to another embodiment of thepresent invention. The out-of-sequence SDU creating and deliveringmethod can be applicable for the RLC layer of the reception end (i.e.,the RLC layer of the UE or that of the base station). Specifically, FIG.14 shows the case where re-transmission of data and reassembling of datablocks in the reception buffer of the RLC layer are processed by usingthe OSD_Window.

With reference to FIG. 14, the RLC layer prepares for datatransmission/reception (DATA_TRANSFER_READY) (S501). The RLC layerreceives a primitive CRILC_HO_START_IND from the RRC layer (S502). TheRLC layer sets an initial value of VR(OH) from the VR(H), a statevariable (S503). The VR(OH) is equal to VR(H)−1. For example, if VR(H)is 8, the VR(OH) becomes 7. An operation of the OSD_Window(Out-of-Sequence Delivery Window) starts to perform out-of-sequence SDUdelivery from the VR(OH).

The RLC layer starts a T_OSD (Timer_Out-of-Sequence Delivery), a timerinforming about termination of the out-of-sequence SDUs (S504). The RLClayer receives PDU, and checks whether the SN of the received PDU iswithin the OSD_Window (S505). If the SN of the received PDU is withinthe OSD_Window, the RLC layer checks whether the received PDU has beenalready stored in the reception buffer (S506).

If the received PDU is not stored in the reception buffer, the RLC layerstores the received PDU in the reception buffer (S507), creates an SDUthat can be reassembled regardless of the order of the received PDU, anddelivers the SDU to the PDCP layer (S508). If the received PDU has beenalready stored in the reception buffer, the RLC layer removes thereceived PDU (S509).

If the SN of the received PDU is not in the OSD_Window (in other words,if the SN of the received PDU is larger than VR(OH)), the RLC layeradvances the OSD_Window such that the VR(OH) becomes the SN of thereceived PDU (S510). The RLC layer stores the received PDU in thereception buffer, and removes PDUs below a lower edge of the OSD_Window(SS511). The RLC layer re-starts the T_OSD (S512). Here, the reason forre-starting the T_OSD is to receive the PDUs within the OSD_Window anddeliver the SDUs out of sequence until the timer expires. Thus, when theOSD_Window advances, it should wait for the T_OSD according to the newOSD_Window.

After re-starting the T_OSD, the RLC layer delivers the SDUs that can bereassembled to the PDCP layer (S508). The RLC layer checks whether there-started T_OSD has expired (8513). If the re-started T_OSD has notexpired, the RLC layer continuously receives PDUs and checks whether SNsof the received PDUs are within the OSD_Window (S505).

If the re-started T_OSD has expired, the PDUs stored in the receptionbuffer are removed and the RLC layer is re-set (S514). Here, inresetting the RLC layer, a reset PDU is not transmitted to thecounterpart RLC layer, so it does not need to transition to a resetpending state. When the resetting is performed, the entire RLC layers,including the RLC layer of the transmission end as well as the RLC layerof the reception end, are re-set. At this time, the RLC layer deliversan SDU transmission stop message indicating that no more SDU will bedelivered to the PDCP layer to the PDCP layer (S515).

Because of the out-of-sequence SDU delivery, the delivery of the datacan be guaranteed without a loss with respect to the SDUs which havebeen received and reassembled at the RLC layer. In addition, because thereassembled SDUs are delivered at a time, signaling for informing thePDCP layer that the out-of-sequence delivery has been completed is notnecessary, and thus, the process gain can be obtained.

FIG. 15 is a view showing a method for creating and delivering SDUs outof sequence in FIG. 14 according to one embodiment of the presentinvention. Here, it is assumed that the SNs of PDUs stored in thereception buffer before the RLC layer receives CRLC_HO_START_IND are 2,6, and 7.

With reference to FIG. 15, the PDUs, data blocks corresponding to theSNs 2, 6, and 7, are stored in the reception buffer, and the size of thereception window (RxWindow) is 8 (SN=1˜8). Here, the state of thereception is that of after receiving the primitive CRLC_HO_START_IND, amessage indicating creation of out-of-sequence SDUs from the RRC layer.The reception buffer may be the reception buffer of the RLC layer of theUE or the reception buffer of the RLC layer of the base station. TheVR(R) and the VR(H) are state variables, and the reception window is aconfigurable parameter. The VR(R) is a lower edge of the receptionwindow, and the VR(H) is an upper edge of the reception window. The sizeof the OSD_Window, the configurable parameter, is the same as or smallerthan the reception window. This is to avoid laying a burden on thebuffer of the reception end RLC layer.

The VR(OH), the state variable, is a value required for OSD_Windowoperation. When the RLC layer receives the CRLC_HO_START_IND, it sets aninitial value of the VR(OH) by using the VR(H) value. For example, theinitial value of the VR(OH) may be set such that VR(OH)=VR(H)−1.

For another example, the initial value of the VR(OH) may be set toindicate the largest one of the SNs of the PDUs received after the RLClayer receives the CRLC_HO_START_IND. The reception buffer stores PDUsof the SNs 2, 6, and 7. In case 1, if the RLC layer receives a PDU ofthe SN 5 after receiving the CRLC_HO_START_IND, the VR(OH) indicates thelargest SN among the SNs of the received PDUs is SN 7. The RLC layerstarts OSD_Window based on the set VR(OH).

The SN of the PDJ received after the RLC layer receivesCRLC_HO_START_IND is 5 and it is within the OSD_Window (namely,VR(OH)≧5(SN)>VR−(Size of OSD_Window)). Thus, the RLC layer stores thePDU of the SN 5 in the reception buffer, decodes segmentation headerinformation (e.g., SI, SO, LI, etc.), and reassembles at least one PDUregardless of the SN to create an SDU. The RLC layer delivers the SDU tothe PDCP layer. Here, an SDU is delivered to the PDCP layer wheneverPDUs are reassembled.

FIG. 16 is a view showing a method for creating and delivering SDUs outof sequence in FIG. 14 according to another embodiment of the presentinvention. It is assumed that the SNs of PDUs stored in the receptionbuffer before the RLC layer receives the CRLC_HO_START_IND are 2, 6, and7. The case shown in FIG. 14 is different from the case shown in FIG. 13in that the SN of a PDU received after the RLC layer receives theCRLC_HO_START_IND is 10 and it is not within the reception window,while, in the latter case (FIG. 13), the SN of the PDU received afterthe RLC layer receives the CRLC_HO_START_IND is within the receptionwindow.

With reference to FIG. 16, the SN of the PDU received after the RLClayer receives the CRLC_HO_START_IND indicating creating of anout-of-sequence SDU from the RRC layer is 10, and it is larger than theupper edge of the OSD_Window. Thus, the RLC layer resets the VR(OH) from7 to 10, and advances the OSD_Window accordingly. If the PDU of the SN10 can be reassembled to create an SDU, an SDU is created and thendelivered to the PDCP layer. Meanwhile, as the PDU of the SN 2 ispositioned outside the lower edge of the OSD_Window due to theadvancement of the OSD_Window is determined not to be reassembled intoan SDU, it is removed from the reception buffer.

As described above, in the handover execution, because the RLC layerdelivers all the possible SDUs to the PDCP layer, re-transmission forrecovering data lost between the UE and the target eNB after handovercompletion can be reduced. Namely, because the amount of control signalsand re-transmission data transmitted to the radio channel to recoverdata of the PDCP layer is reduced, the overhead of radio resources canbe reduced, the recovery time with respect to the lost data can beshortened, and the final throughput can be improved.

In addition, in a state that data transmission/reception is availableafter the RLC layer is reset, if handover is completed, the RLC layercan be released, and even if handover fails, an internal signalingindicating a start of data transmission/reception between the UE and thesource eNB is not necessary. Also, because a control signal in a radiochannel for matching synchronization between RLC layers of the both endsis not necessary, a transmission delay can be reduced and a processinggain can be obtained.

In the previously described embodiments, by not flushing buffers duringhandover as done in the prior art, and by sending out-of-sequence SDUs,the RLC layers of the UE and eNB that were established prior to handoverare re-established after handover, rather than completely recreated.

Thus, in the present invention, for both a UM and an AM RLC entity,where possible, RLC SDUs are reassembled from lower layer PDUs anddelivered to an upper layer (e.g., to the PDCP layer) in a sequence.This reassembly may include restricting the reassembly to PDUs with aserial number less than a threshold value and may include removingcorresponding RLC headers.

The foregoing description of the preferred embodiments of the presentinvention has been presented for the purpose of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed, and modifications andvariations are possible in light of the above teachings or may beacquired from practice of the invention. It is intended that the scopeof the invention be defined by the claims appended hereto and theirequivalents.

What is claimed is:
 1. A method of performing handover by a mobileterminal from a source base station to a target base station, the mobileterminal having a Radio Resource Control (RRC) layer and a Radio LinkControl (RLC) layer, the method comprising: receiving, at the RRC layer,a handover command; receiving, at the RLC layer, an indicationassociated with the handover command; upon receiving the indication,reassembling, at the RLC layer, at least one RLC service data unit (SDU)from at least one protocol data unit (PDU) having a sequence number thatis less than a variable, wherein the variable indicates a sequencenumber following a highest sequence number among at least one PDUreceived out of sequence by the RLC layer; delivering, at the RLC layer,the reassembled at least one RLC SDU to an upper layer of the mobileterminal; and discarding, at the RLC layer, at least one remaining PDUthat could not be reassembled into at least one RLC SDU.
 2. The methodof claim 1, further comprising: checking header information of the atleast one PDU having the sequence number.
 3. The method of claim 1,wherein the indication is a handover primitive which indicates a startof the handover.
 4. The method of claim 1, further comprising: uponreceiving the indication, stopping, at the RLC layer, a data receipt. 5.The method of claim 1, further comprising: upon receiving theindication, stopping, at the RLC layer, a data receipt; and reordering,at the upper layer, the reassembled at least one RLC SDU, wherein theupper layer is a Packet Data Convergence Protocol (PDCP) layer.
 6. Amobile terminal, comprising: a Radio Resource Control (RRC) layer; aRadio Link Control (RLC) layer; a radio frequency (RF) unit configuredto receive and transmit a signal; and a processor coupled to the RF unitand configured to: receive, at the RRC layer, a handover command;receive, at the RLC layer, an indication associated with the handovercommand; upon receiving the indication, reassemble, at the RLC layer, atleast one RLC service data unit (SDU) from at least one protocol dataunit (PDU) having a sequence number that is less than a variable,wherein the variable indicates a sequence number following a highestsequence number among at least one PDU received out of sequence by theRLC layer of the mobile terminal; deliver, at the RLC layer, thereassembled at least one RLC SDU to an upper layer of the mobileterminal; and discard, at the RLC layer, at least one remaining PDU thatcould not be reassembled into at least one RLC SDU.
 7. The mobileterminal of claim 6, wherein the processor is further configured tocheck header information of the at least one PDU having the sequencenumber.
 8. The mobile terminal of claim 6, wherein the indication is ahandover primitive which indicates a start of the handover.
 9. Themobile terminal of claim 6, wherein the processor is further configuredto stop, at the RLC layer, a data receipt upon receiving the indication.10. The mobile terminal of claim 6, wherein the processor is furtherconfigured to: upon receiving the indication, stop, at the RLC layer, adata receipt; and reorder, at the upper layer, the reassembled at leastone RLC SDU, wherein the upper layer is a Packet Data ConvergenceProtocol (PDCP) layer.